1. Field of the Invention
This invention relates to a dip forming apparatus for accreting molten metal on a moving core wire to form a cast rod and particularly to such an apparatus best suited for forming a cast rod composed of a core wire of one metal and a thin clad of another metal formed around the core wire.
2. Prior Art
A conventional dip forming apparatus 10 shown in FIG. 1 comprises a furnace 12 for holding molten metal M, a crucible 14 of graphite connected to the furnace 12 via a conduit 16, a crucible housing 18 of a refractory material enclosing the crucible 14, an electric heater element 20 wound around the crucible 14, a hermetic housing 22 connected to a bottom of the crucible housing 18, and a cooling housing 24 defining a cooling chamber and connected to the top of the crucible housing 18. A bushing 26 is fitted in a hole 27 formed through a bottom wall of the crucible 14 and is also connected to a bottom wall of the crucible housing 18. A pulley 28 is mounted within the hermetic housing 22, and three pairs of pinch rolls 90 are also mounted within the hermetic housing 22. The furnace 12 has a pressure portion 30 and a supply portion 32 connected together at their lower ends by a connecting portion 34.
A core wire W of metal is adapted to pass through the hermetic housing 22, the bushing 26 and the crucible 14. The pinch rolls 90 serves to straighten the core wire W and to move it upwardly toward the crucible 14. Thus, the core wire W is continuously fed upwardly through a bath of molten metal M in the crucible 14, so that the molten metal accretes on the outer surface of the core wire W to provide a cast rod R, the accreting metal on the core wire W solidifying to form a clad of the cast rod R.
A level sensor 36 is provided for sensing the level of the molten metal in the supply portion 32 of the furnace 12. A conduit 38 connects the pressure portion 30 of the furnace 12 to a source (not shown) of reducing gas under pressure via a pressure regulator of the diaphragm type (not shown). The supply portion 32 of the furnace 12 has a feed port 40 for selectively supplying molten metal to the supply portion 32 from a supply source (not shown). The pressure regulator is controlled in accordance with the sensing signal from the level sensor 36, so that the pressure in the pressure portion 30 of the furnace 12 is controlled by the pressure regulator so as to keep the molten metal in the supply portion 32 at a predetermined level. The level of the molten metal in the pressure portion 30 is lower than that of the molten metal in the supply portion 32. It will be appreciated that the molten metal in the crucible 14 and the molten metal in the supply portion 32 are at the same level. It is important that the molten metal in the supply portion 32 should be maintained at the predetermined level in order to ensure that the clad of the resulting cast rod R has a uniform thickness along the length thereof. As the accreting operation proceeds, the molten metal is consumed, and the pressure in the pressure portion 30 is increased gradually to keep the molten metal in the supply portion 32 at the predetermined level.
Recently, a cast rod composed of a core wire of one metal and a clad of another metal has been produced, using the above conventional dip forming apparatus. For example, a cast rod composed of a steel core wire and a copper clad has been produced, and particularly it has recently been desired to produce such a cast rod having a thin copper clad, that is to say, a low ratio of the copper clad to the cast rod in cross-sectional area. In the production of a cast rod having such a low area ratio, the level or depth of the molten metal in the crucible 14 is usually several tens of millimeters, and it is necessary that a deviation from the predetermined level of the molten metal in the crucible 14 should be maintained within a range of plus or minus several millimeters in order to ensure that a variation in amount of accretion of molten metal around the core wire in its longitudinal direction is kept to a minimum. The reason is that the variation in accretion amount will invite a variation in electrical conductivity and tensile strength of the resultant cast rod.
The reducing gas is introduced into the pressure portion 30 under the control of the pressure regulator of the diaphragm type so as to increase the pressure in the pressure portion 30, and a pressure variation can be kept at best to .+-.0.01 kgf/cm.sup.2. Therefore, a deviation of the level of the molten metal in the crucible 14 is about .+-.11 mm as indicated below: EQU 10000 mm/8.9 (specific gravity of copper).times.0.01=11 mm
Thus, with the conventional dip forming apparatus, it has been rather difficult to produce a cast rod having a relatively low cross-sectional area ratio of the copper clad to the cast rod and having substantially constant electrical conductivity and tensile strength. Incidentally, in the production of a cast rod of the type in which the ratio of the copper clad to the cast rod in cross-sectional area is relatively high, for example, 60 to 70%, the level or depth of the molten metal in the crucible is usually several hundreds of millimeters, and a deviation of this level resulting from a change in the pressure of the pressure portion 30 is relatively small with respect to the molten metal level in the crucible. Therefore, in such a case, the level variation in the crucible has not adversely affected the ratio of the clad to the cast rod.
For a better understanding of the conventional dip forming apparatus, FIG. 2 shows a relation between a time of dipping of the core wire in the crucible and a ratio of the accreted metal on the core wire to the cast rod (i.e., the cross-sectional area ratio of the copper clad to the cast rod). If a low area ratio of 20 to 30% is desired, the dipping time need to be not more than one tenths of the dipping time required for the conventional cast rod having an area ratio of 60 to 70%, and therefore the depth of the molten metal in the crucible need to be not more than one tenths. A distribution of the area ratio of the cast rod in its longitudinal direction is shown in a broken line in FIG. 3. As is clear from FIG. 3, a variation in the area ratio is relatively large. The reason why the area ratio is subjected to such a large variation is that a deviation of the level of the molten metal in the crucible is relatively large with respect to this level.